Mike the Strike wrote:
"GC flight logs contain a GPS time stamp which is the most exact time
standard readily available. So, violations of this requirement are
quite plain to see in these logs."

Unfortunately, sunset doesn't come with a GPS timestamp, so I'm afraid
this isn't as easy as you make it sound.

Yes, I know there are standard tables of sunset times, but these make
assumptions that may not be exactly correct and at a given location
actual sunset may differ from the calculated one by many minutes.
(Ask any local physicist./astronomer).

I am both of those and I assure you that the time of sunset is well-
defined and readily predicted for any location to a tiny fraction of
a second.

You may have difficulty observing the position of the sun. Like,
it might be cloudy, there might be a mountain in the way, or
you might be indoors. But that is not relevent.

Any regulatory requirement should (and I daresay will) be based
on that well-defined time, or the equally well-defined times of civil,
nautical, or astronomical twilight.

http://en.wikipedia.org/wiki/Astronomical_twilight
http://aa.usno.navy.mil/faq/docs/RST_defs.html#top

Obviously you may be safer flying a few minutes after sunset in
a clear sky than a few minutes before sunset in a cloudy sky.
But that is a practical consideration.

You are correct that these definitions are based on certain
assumptions, but the point is that a rule that says you are
supposed to be on the ground by sunset, the end of civil
twilight, or whatever, should be a rule that uses the defined
time, not some local observation. That gives you a clear,
unambiguous requirement.

--

FF